I have a freezer that I use for temp control. However, it's been unplugged all summer and its growing mold. Is this going to be harmful to my respiratory system for me to open and clean it? What precautions should I take?

Dust masks are relatively effective: you should be able to find one that goes to less than a micron for the smallest size particle it filters. I've seen 0.3 microns for the smallest effective size. Like here:
http://www.filtera.com/3mmod8662esh.html

Another factor to consider any time you are cleaning up a hazard (whether mouse poop or mold) is to wet the dust first. I've read suggestions to use chlorox bleach spray to saturate mouse poop. Why? Wet particles aren't stirred up as easily, wet particles don't break into even smaller parts. Presumably the chlorox bleach spray also helps kill bacteria or mold. You don't want to stir this stuff up and end up having it in your house duct system where your kids etc will breathe it. If you can roll it outside first, I would do that to limit how much you get in the house.

Why not just pick up a dust mask for ~$5 or $10 (useful for other household work too) to limit what you inhale? And use some chlorox spray to wet down the mold before your disturb it.

This advice isn't based on expertise or training just personal experience and some limited web research.

In terms of personal experience, I cleaned the mold out of a garage fridge that had food in it that got disabled (breaker intentionally flipped by the electrician). Mold city. 409, Chlorox bleach spray, rubber gloves, dust mask. It worked for me. However, your mileage may vary.

@Keith Hoffman has some great info. If you do want to take precautions, a dust mask would be one of the most beneficial items. Typical dust masks are rated as an "N95" mask. The number "95" corresponds with industrial filter ratings. An N95 is 95% efficient at removing 0.3 micron particles. 0.3 micron particles are considered the most difficult particle to filter out. Different manufacture's mask will have slightly different effective filtration ranges depending on the media (material) type. Typically mold spores are large enough to be filtered out by an N95 mask. If you want to be very sure, a P100 type filter is effectively a HEPA filter. P100 masks will be at least 99.9% efficient at removing 0.3 micron particles. I wouldn't waste your money on a P100, unless your using an actual respirator. A dust mask is only as effective as the seal it makes on your face. Most dust mask leave a GIANT hole where the mask goes over your chin. Companies spend thousands of dollars to have a dust masks "fit tested" to ensure they are working properly. Gloves would be a good idea, however mold spores will not enter your body through your skin, soap and water will wash the spores away. A spray of some kind should be used to wet the spores before cleaning. A bleach spray (mixed as directed on the jug) is great, however, even water in a spray bottle will effectively "encapsulate" the spore and keep it from floating around everywhere. Once you've cleaned the mold up, make sure to disinfect the area. After disinfection has occurred (ie: 20 min if using a properly mixed bleach solution) dry the area so that there is no moister left over. You will NOT be able to completely removed/ kill every last spore, so you want to make sure the environment in your freezer is not conducive to new growth via old spores.